Foundry molds and cores and process for making same



1952 R. G. DUNN ET AL 3,059,297

ESS FOR MAKING SAME fiafierf f zfin i BY 7202720.; 73 736%? Patented Oct. 23, 1962 3,059,291 FOUNDRY MOLDS AND C(PRES AND PRQCESS FOR MAKING SAME Robert G. Dunn, Birmingham, and Thomas B. Pfafi, Auburn Heights, Mich, assignors to General Motors CorporatiomDetroit, Micln, a corporation of Delaware Filed Nov. 12, 1959, Ser. No. 852,311 7 Claims. (Cl. 22193) This invention relates to foundry molds and cores for use in metal casting operations and more particularly to dispensable molds and cores having a resin binder.

Among the techniques presently employed in foundry practice for the production of precision or semi-precision castings are those in which resinous binders are used to hold together the sand particles of which the mold. or cores are made. In general these molds and cores involve the use of thermosetting binders such as phenolformaldehyde resin for the sand grains to form thin-walled molds or cores having a high gas permeability, good surface smoothness and dimensional stability. The molding material generally consists of a dry mixture of a major proportion of sand and a minor proportion of a thermosetting binder used in powder form or as a coating on the sand particles. Molds are prepared byallowing the dry molding mixture to come into contact with a hot metal pattern heated to a temperature on the order of 350 F. to 600 F. for a period of time ranging from a few seconds to about a minute until a desired mold thickness is built up. Thereafter the excess molding material is removed and the closely adhering sand-resin layer is cured by heating the mold while in contact with the metal pattern at a temperature ranging from 300 F. to as high as 1500 F. for a time sufiicient to cure the resin into a hard and infusible material.

While the molds and cores made in this manner are satisfactory, they are relatively expensive because the phenol-formaldehyde thermosetting resins normally used are relatively expensive, and because considerable time and expensive equipment are necessary to cure the resin. Moreover, this process is not satisfactory for making relatively thick mold or core sections because of the excessive time required to properly cure the resin binder.

In another process disclosed in the co-pending application Serial No. 726,120, filed September 19, 1958, now U.S. Patent No. 3,008,205, granted November 14, 1961, and assigned to the assignee of the present invention, an acid gas curable binder is admixed with the sand. After the sand-resin mix is blown or otherwise deposited in the mold or core forming cavity, the acid gas such as hydrogen chloride or chlorine is forcibly passed through the sand binder mass to catalyze and rapidly cure the binder. This provess is advantageous in that the resin in the molding mixture is cured in a very short time of about 3 to 4 seconds. However, this process introduces the problem of handling the highly toxic acid gas in a manner so as not to endanger the health of the workmen.

It is a basic object of the present invention to provide a method for making dispensable, gas permeable sandresin type molds and cores which takes advantage of the rapid and efficient acid cure of the binder and which substantially eliminates the need for handling gaseous hydrogen chloride, chlorine or the like. It is a further object of this invention to provide a sand binder mixture which may be efficiently cured by an acid catalyst without the need for handling toxic acid gase or acid and to provide molds and cores made therewith.

These and other objects are accomplished by providing a binder mixture consisting of an aqueous solution of about 30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water and 10 to 30 parts by weight of furfuryl alcohol. This binder mixture is then admixed with 0.5% to 3.0% by weight of ammonium chloride based on the Weight of the binder mixture. Preferably the ammonium chloride is first dissolved in water in an amount up to 10% by weight of the binder mixture. The resulting binder-ammonium chloride mixture is then mixed thoroughly with a foundry sand in an amount ranging from about 1% to 5% of the weight of the sand. The sandbinder mix thus formed is then blown or otherwise deposited in the mold cavity defining the contour of the foundry mold or core to be made. Preferably the core box is heated to a temperature of about 400 F. so that as the sand binder mix is blown into the mold cavity, the heat imparted thereto is effective to cause the evolution of hydrochloric acid through the sand-resin mix to efficiently and rapidly cause the binder components to form a hard, infusible, furfuryl alcohol modified urea-formaldehyde resin. Alternatively, the sand-resin mix may be blown into a cold mold cavity or core box to form the mold or core and thereatfer a gas such as air or carbon dioxide heated to about 400 F. is passed through the core binder mix which is also effective in causing the evolution of the hydrogen chloride gas to efficiently and rapidly cure the binder components.

Other objects and advantages of this invention will more fully appear from the following detailed description of a preferred embodiment thereof, reference being bad to the drawings wherein apparatus suitable for practicing the process is shown.

In the drawings:

FIGURE 1 shows a core box associated with a blowhead for applying the molding mixture into the core box; and

FIGURE 2 illustrates a means for blowing hot air into thefilled core box cavity.

The invention will now be described in greater detail in terms of specific examples of the process for forming a core. It will be obvious, however, that the method described is equally applicable for making foundry molds. A preferred binder, identified herein as composition 1, capable of undergoing a condensation reaction in the presence of an acid gas, is prepared which consists by weight of 42.0 parts formaldehyde, 47.5 parts urea, 10 parts Water and 20 parts furfuryl alcohol to which has been added 1.2 parts ammonium chloride dissolved in 6 additional parts of water. A molding mixture is then prepared by thoroughly admixing parts by weight of lake sand and 2 parts by weight of the above-described liquid binder to distribute the binder thoroughly through the sand particles.

As shown in FIGURE 1, a core box is provided which consists of a lower half 10 positioned on a suitable sup port 11 and an upper half 12. A seal 14 is provided for sealing the core box halves. when they are in a closed position. The upper core box half includes a centrally disposed opening 16 and side vent openings 18. Disposed over the upper core box half is a magazine 20' carried by the support 21. The sand magazine 20 is attached to a blow plate 22 having a blow tube 24 which extends to the surface of the core box cavity through the opening 16 when the blow plate is positioned on the upper core box half 12. The core box cavity is filled by applying compressed air to the sand-resin magazine through the valve 26. The vents 153 permit escape of the compressed air. After the cavity is filled, the blow plate 22- is removed from the core box whereby the molded sandresin molding mixture extends only to the surface of the core box cavity. Screen members 19 associated with the vent openings 18 prevent the molding mixture from being blown into the vent openings.

Next, as shown in FIGURE 2, a plate 28 is placed over the upper half of the core box 12. The plate 23 includes a conduit 30 in alignment with the core box opening 16 for admitting hot air under pressure to the core box cavity. The plate 28 also includes exhaust conduits 32 leading from the vents 18. Hot air in the neighborhood of 400 F. and under pressure of about 30 pounds per square inch gauge is injected through the formed core with the result that hydrogen chloride is formed throughout the core which is effective in curing the core in about seconds. Preferably a vacuum is applied to the conduit 32 for exhausting any residual acid gas which may remain in the core.

In an alternate procedure, the core box halves of FIG- URE 1 are provided with heating elements (not shown) preferably of the electrical resistance type embedded in the core box walls, and the core box is thereby maintained at a temperature ranging from about 350 F. to about 600 F. and preferably about 400 F. The sand-resin molding mixture is blown into the hot core box cavity as shown in FIGURE 1. As the molding mixture strikes the hot core box walls, hydrogen chloride is released which initiates a cure of the binder components during the blowing process. The sand-resin blowing process is completed in a second or two and only 3 to 4 more seeonds are required to convert the binder into a hard, infusible and insoluble resin with a resulting rigid, relatively hard core which may be readily removed from the core box without damage. Molds and cores formed by either of these procedures were found to have a highly satisfactory hardness and tensile strength.

Molds and cores were made utilizing the following binder composition 11.

Parts by weight Formaldehyde 54 Urea 53.4 Furfuryl alcohol Water 12.6

Formaldehyde Urea 42.5 Water 7.5 Furfuryl alcohol 20 One part of ammonium chloride was dissolved in 5 parts of water and admixed with the above ingredients. This mixture was then thoroughly mixed with lake sand in an amount equal to 2% of the weight of the sand. Cores and molds prepared with this sand-resin mix were found to have a satisfactory tensile strength and were otherwise found suitable for general foundry use.

The manufacture of satisfactory cores and molds in accordance with the present invention is dependent on the control of several factors of a more or less critical nature. The control of ratio of formaldehyde to urea in the binder mixture, the use and quantity of furfuryl alcohol, the water content and the catalyst content are all essential. As has been illustrated above, it is highly desirable to maintain the formaldehyde content between 30 parts and 54 parts and the urea content between 42 parts and 54 parts. While it is possible to make usable cores and molds without the use of furfuryl alcohol, its use in amounts ranging from about 10 parts to 30 parts in the above compositions I, II and III is considered essential to a commercially successful process. The furfuryl alcohol provides the final binder mixture with a suitable extended shelf-life and. the resulting core or mold with a markedly improved tensile strength. Thus, cores were prepared using the above preferred composition I wherein the furfuryl alcohol content was varied in amounts of zero, 10 parts, 20 parts, 30 parts and 40 parts. Cores from each of these compositions were found to have a tensile strength of 170, 275, 425, 500 and 180 pounds, respectively, per square inch. It is thus seen that markedly superior cores are obtained using from about 20 to about 30 parts of furfuryl alcohol.

The control of the water content is also very important in the production of superior cores and molds in accordance with the present invention. The composition I was varied in that the ammonium chloride was mixed into the binder solution in a dry state in one experiment and dissolved in about 12 parts of water in another experiment. Cores made from sand-resin mixes using each of these binders, though satisfactory, were found to have a one-third lesser tensile strength than those made from the composition I.

As set forth above, the practical limit in the use of the ammonium chloride catalyst is between about 0.5% and 3.0% of the weight of the binder. When less than about 0.5% ammonium chloride is used, the curing rate is too slow for practical production purposes whereas if more than 3% of the ammonium chloride is used, the shelf-life is excessively short. Suitable reaction rates and adequate shelf-life of the binder are obtained when the ammonium chloride is present in amounts ranging from 1% to 2% of the weight of the binder. Although the use of ammonium chloride is preferred, other acid salts such as oxalic acid and ammonium sulfate may be used.

In instances where it is desired to achieve a high rate of cure and it is desired to use a relatively high proportion of ammonium chloride such as in excess of 2%, the shelf-life of the binder mixture may be increased by the addition of small amounts of hexamethylenetetramine, up to about 5% by weight of the binder mix which serves to buffer the binder mixture.

In general, satisfactory cores and molds may be made using a sand-resin mix in which the resin portion comprises about 1% to 5% of the weight of the sand. In this sense, the term sand is meant to refer to the various foundry sands such as lake sand and Juniata sand and also to other suitable comminuted refractory substances such as silica flour or similar fine-facing materials which may be incorporated in the sand to provide molds or cores with especially smooth working surfaces as desired.

Conveniently the urea and formaldehyde portions of the binder mix may be mixed by adding a suitable proportion of urea to the commercially available concentrates consisting of about 25% urea, 60% formaldehyde and 15% water.

The process of the present invention produces cores and molds which are markedly superior to those made by the prior art techniques involving a urea-formaldehyde binder cured by means of heat. The process thus makes it possible on a commercial basis to use cheap urea-formaldehyde binders in the manufacture of cores or molds. Heretofore, the use of urea-formaldehyde binders has been suggested but never, insofar as it is known, used on a practical commercial basis since such cores and molds were regarded as being excessively weak for general application,

While the present invention has been described by means of certain specific examples, it will be understood that the scope is not limited thereby except as defined in the following claims.

We claim:

1. A molding mixture consisting essentially of about parts by weight of sand and about 1 to 5 parts by weight of a potentially reactive binder mixture and an acid salt capable of catalyzing the reaction of said mixture on being subjected to heat at elevated temperatures, said mixture consisting essentially of about 30* to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water and about 1-0 to 30 parts by weight of furfuryl alcohol, said acid salt being present in an amount of about 0.5% to 3% of said binder mixture dissolved in water in an amount up to about by weight of said binder mixture.

2. A molding mixture consisting essentially of about 100 parts by weight of sand and about 1 to 5 parts by weight of a potentially reactive binder mixture and an acid salt capable of catalyzing the reaction of said mixture on being subjected to heat at temperatures in the neighborhood of 350 F. to 600 F., said mixture consisting essentially of about 30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by Weight of urea, about 7 to 13 par-ts by weight of water and about 10 to 30 parts by weight of furfuryl alcohol, said acid salt consisting of about 0.5 to 3%of ammonium chloride based on the weight of said binder mixture dissolved in water in an amount up to about 10% by weight of said binder mixture.

3. A method of producing sand-resin molds and cores which comprises forming a molding mixture consisting essentially of about 100 parts by weight of sand and about 1 to 5 parts by weight of a potentially reactive binder mixture and an acid salt capable of catalyzing the reaction of said mixture on being subjected to heat, said binder mixture consisting essentially of about '30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water and about 10 to 30 parts by weight of furfuryl alcohol, said acid salt consisting of about 0.5 to 3% of the weight of said binder mixture dissolved in water in an amount up to about 10% by weight of said binder mixture, the constituents of said binder mixture being present in said molding mixture in substantially an unreacted state depositing said molding mixture within a mold cavity, subjecting said mixture to heat in the order of 350 F. to 600 F. for a time sufficient to cause said acid salt to catalyze the reaction of said binder mixture in situ and to cause said binder to harden, and finally removing the resulting article from said cavity.

4. A method of producing sand-resin molds and cores which comprises forming a molding mixture consisting essentially of about 100 parts by weight of sand and about 1 to 5 parts by weight of a potentially reactive binder mixture and an acid salt capable of catalyzing the reaction of said mixture on being subjected to heat,

a said mixture consisting essentially of about 30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water and about 10 to 30 parts by weight of fu'rfuryl alcohol, said acid salt consisting of about 0.5% to 3% ammonium chloride based on the weight of said binder mixture dissolved in water in an amount up to about 10% by weight of said binder mixture, the constituents of said: binder mixture being present in said molding mixture in substantially an unreacted state blowing said molding mixture into a mold cavity heated to a temperature on the order of 350 F. to 600 F. for a time sufiicient to cause said ammonium chloride to catalyze the reaction of said binder mixture and the binder to harden, and finally removing the resulting article from said cavity.

5. A method of producing sand-resin molds and cores which comprises forming a molding mixture consisting essentially of about parts by weight of sand and about 1 to 5 parts by weight of a potentially reactive binder mixture and an acid salt capable of catalyzing the reaction of said mixture on being subjected to heat, said mixture consisting essentially of about 30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water and about 10 to 30 parts by weight of furfuryl alcohol, said acid salt consisting of about 1% to 2% ammonium chloride based on the weight of said binder mixture dissolved in water in an amount up to about 10% by weight of said binder mixture, the constituents of said binder mixture being present in said molding mixture in substantially an unreacted state blowing said mixture within a mold cavity, subjecting said molding mixture to a gas heated to a temperature of 350 F. to 600 F. for a time sufficient to cause said acid salt to catalyze the reaction of said mixture and the binder to harden, and finally removing the resulting article from said cavity.

6. A method of producing sand-resin molds and cores which comprises blowing a moldingmixture into a hot mold cavity, the walls of said mold cavity being heated to a temperature on the order of 350 F. to 600 F., said molding mixture consisting essentially of about 100 parts by weight of sand and about 1 to 5 parts by weight of a mixture consisting essentially of about 30 to 54 parts by weight of formaldehyde, about 42 to 54 parts by weight of urea, about 7 to 13 parts by weight of water, about 10 to 30 parts by weight of furfuryl alcohol and about 0.5% to 3% ammonium chloride based on the weight of the binder dissolved in water up to about 10% of the weight of the binder, maintaining said mixture in said hot mold for a few seconds suilicient to cause said ammonium chloride to promote the reaction of said binder constituents and cause them to harden, and removing the resulting article from said cavity.

7. A mold or core formed by heating a molding mixture consisting essentially of about 100 parts by weight of sand and about 1 to 5 parts by weight of a binder consisting essentially of about 30 to 54 parts by weight of formaldehyde, about 4 2 to 54 parts by weight of urea, about 7 to 13 parts by weight of water, about 10 to 30 parts by weight of furfuryl alcohol, and about 0.5 to 3% of an acid salt based on the weight of the binder mixture dissolved in water up to about 10% of the weight of said binder capable of catalyzing the binder mixture under the influence of heat.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,168 Staeger Dec. 23, 1947 2,471,600 Adams et al. May 31, 1949 2,521,614 Valyi Sept. 5, 1950 2,683,296 Drumm et al. July 13, 1954 2,721,363 Taylor Oct. 25, 1955 2,923,989 Thompson Feb. 9, 1960 FOREIGN PATENTS 63 1,109 Great Britain Oct. 27, 1949 215,309 Australia June 9, 1958 573,760 Canada Apr. 7, 1959' 

1. A MOLDING MIXTURE CONSISTING ESSENTIALLY OF ABOUT 100 PARTS BY WEIGHT OF SAND AND ABOUT 1 TO 5 PARTS BY WEIGHT OF A POTENTIALLY REACTIVE BINDER MIXTURE AND AN ACID SALT CAPABLE OF CATALYZING THE REACTION OF SAID 